1. Field of the Invention
The present invention relates to a method of fabricating a near-field light-generating element for shining or detecting near-field light used in a near-field optical microscope or near-field optical memory device and, more particularly, to a method of fabricating a near-field light-generating element having a minute scattering body inside an aperture.
2. Description of the Related Art
Scanning probe microscopes (SPMs) typified by scanning tunneling microscopes (STMs) and atomic force microscopes (AFMs) are used to observe microscopic regions on the nanometer order on sample surfaces. In SPM, a probe having a sharpened tip is scanned across a sample surface. An interaction such as a tunneling current or atomic force produced between the probe and the sample surface is taken as a subject to be observed. An image of a resolution dependent on the topography of the probe tip can be obtained. However, relatively strict limitations are imposed on observed samples.
Accordingly, a scanning near-field optical microscope (SNOM) attracts attention today. The microscope takes an interaction produced between near-field light produced at the tip of a probe and a sample surface as a subject to be observed to thereby permit observation of microscopic regions on the sample surface.
In SNOM, near-field light is shone onto a sample surface from an aperture formed at a sharpened tip of optical fiber. The aperture has a size of less than the diffraction limit of the wavelength of light introduced into the optical fiber. For example, it has a diameter of about 100 nm. The distance between the aperture formed at the tip of the probe and the sample is controlled by SPM techniques. The value is less than the size of the aperture. At this time, the spot diameter of the near-field light on the sample is almost equal to the size of the aperture. Therefore, optical properties of a sample in a microscopic region can be observed by scanning the near-field light impinging on the sample surface.
Such a near-field light-generating element can be applied as a high-density optical memory device which creates near-field light of high energy density in the aperture portion of a probe by introducing light of relatively large intensity toward the sample through the probe and locally modifying the structure or physical property on a recording medium surface by the near-field light. The optical memory device can also be used as a microscope. Attempts have been made to increase the angle at the tip of the probe in order to obtain near-field light of large intensity. Furthermore, in applications of such a memory device, some devices where a probe having an aperture in a flat substrate unlike a sharpened probe is used as a record/read head have been devised.
In these elements making use of near-field light, formation of an aperture is important. As one method of forming the aperture, a method disclosed in patent publication No. 21201/1993 is known. In the method of forming an aperture of patent publication No. 21201/1993, a sharpened optical waveguide on which a light-shielding film is deposited is used as a sample for forming an aperture. The method of forming the aperture consists of pressing the sharpened optical waveguide having the light-shielding film thereon against a hard flat board with a quite small amount of push that is controlled well by a piezoelectric actuator to thereby plastically deform the light-shielding film at the tip.
Another method of forming an aperture is disclosed in patent laid-open No. 265520/1999. In the method of forming an aperture in patent laid-open No. 265520/1999, the subject in which an aperture is formed is the tip of a protrusion formed on a flat board or plate by a focused ion beam (FIB). The method of forming the aperture is carried out by directing an FIB onto a light-shielding film at the tip of the protrusion from a side and removing the light-shielding film at the tip of the protrusion.
In addition, in order to improve the resolution and to increase the intensity of scattering light produced as a result of an interaction created between the probe and the sample, a method making use of a phenomenon where fine metal particles are made to produce plasmons by incident light has been proposed.
Okamoto et al. have proposed a probe having fine particles of a metal such as Au (gold) or Pt (platinum) fixed at the tip of a probe body that is made of a transparent material such as SiN (silicon nitride) and is a sharp, tapering member (Takayuki Okamoto and Ichirou Yamaguchi, xe2x80x9cNear-field scanning optical microscope using a gold particlexe2x80x9d, Jpn. J. Appl. Phys. 36, L166 (1997)).
In such a probe where the metal fine particles are fixed at the tip of a sharpened probe body made of a transparent material, the metal fine particles are made to produce plasmons by incident light. The scattering efficiency is higher compared with the prior art probe having no metal fine particles. A larger amount of detected light can be obtained. Since the resolution is determined by the position at which metal fine particles are fixed at the tip, the radius of curvature, the kind of the metal fine particles, and so on. Therefore, a higher resolution can be derived by fixing appropriate fine metal particles to the tip of a probe.
Furthermore, according to the optical fiber probe and method of fabricating same as disclosed in U.S. Pat. No. 3,117,667, a protruding portion of a core protruding from a clad is formed at one end of optical fiber. A metal film is formed on the surface of the protruding portion except for the front-end portion. The outer portion of the protruding portion is made to recede from the front-end surface. A metal sphere is formed at the tip of the inner portion. Therefore, an optical fiber probe can be obtained which can detect near field at high sensitivity and high resolution without being affected by scattering light scattered by the base portion of the probe or by scattering light due to the surface roughness of the sample.
In addition, according to the method of creating metal fine particles at the tip of a member, fixing the particles, apparatus therefor, and probe disclosed in patent laid-open No. 2001-83069, a method of forming a metal sphere at the tip of a probe by immersing a sharp member in a metal solution and deoxidizing ions by near-field light, a probe, and apparatus are provided.
Generally, narrowing the aperture lowers the intensity of near-field light produced near the aperture. Where this is scattered or modulated by a sample (or recording medium), the intensity of modulated and propagating light reaching the detector drops. In order to compensate for this, even if the gain of the detection system is increased, the signal-to-noise ratio (S/N) rather deteriorates considerably because of dark current intrinsic to the detector and thermal noise in the amplifier circuit. Of course, increase in the power of laser light introduced into the aperture portion and decrease in the optical spot of laser focused in the aperture portion are advantageous.
However, as the aperture is reduced in size, the thickness of the light-shielding film is urged to be reduced because of restrictions on the micromachining using an FIB or the like and by the effects of attenuation of the introduced light dependent heavily on the ratio between the in-plane dimensions of the aperture and the thickness. Therefore, thinning of the light-shielding film deteriorates the light shielding and increases the dc light component reaching the optical detector. If comparable optical intensity modulation is obtained due to the sample (or recording medium), equivalent signal quality deterioration results. Also, where light is collected using a lens near the aperture portion, the geometrical optics is fundamentally based on the prior art geometrical optics. Consequently, it is impossible to shine light onto the vicinities of the aperture portion at a sufficiently high energy density due to the diffraction limit.
Accordingly, with the conventional method, it is quite difficult to achieve improvement of the reproduced resolution and higher S/N of the signal obtained from the optical detector simultaneously. There is the problem that it is difficult to increase the recording (reading) density.
As a countermeasure against it, in inventions disclosed by Okamoto et al. in U.S. Pat. No. 3,117,667 and patent laid-open No. 2001-83069, a metal fine particle for producing plasmons is formed at the tip of a sharpened probe.
Although such probes are capable of improving the resolution and increasing the light intensity and the S/N obtained with a detector, these probe shapes are not adapted for mass production. Expensive fabrication equipment is necessary to fabricate near-field light-generating elements. There is the problem that it is difficult to reduce the costs of SNOM devices and optical memory devices.
It is an aspect of the present invention to provide near-field light-generating elements which produce sufficiently large read signal intensity and S/N, ultrahigh density, ultrahigh resolution but can be mass-produced at a low cost.
Accordingly, in order to solve the foregoing problems, a method of fabricating a first near-field light-generating element according to the present invention is adapted to fabricate a near-field light-generating element having a minute scattering body inside an aperture, the scattering body producing plasmons by being illuminated with light, said method comprising the steps of: forming minute scattering bodies placed periodically on a substrate; selecting only one of the minute scattering bodies placed periodically and removing the other minute scattering bodies; and forming said aperture such that said one minute scattering body is placed inside said aperture.
This invention makes it easy to fabricate a near-field light-generating element having a minute scattering body for producing plasmons inside an aperture. The fabrication process for the near-field light-generating element according to the invention itself can be effected in a semiconductor process. High-performance near-field light-generating elements making use of plasmons can be mass-produced. In addition, the cost can be reduced easily. Moreover, similar semiconductor processes can be used as processes for fabricating shapes of near-field light-generating elements other than the aperture portion necessary to apply the present near-field light-generating elements to near-field optical microscopes or optical memory devices. Therefore, the process compatibility between the aperture formation step and the step for forming shapes of other portions is very high.
Furthermore, the efficiency of utilization of light can be immensely enhanced by using a near-field light-generating element fabricated by the fabrication method according to the invention and producing plasmons in a microscope or optical information record/read device. The power consumption can be reduced. The device can be miniaturized.
A method of fabricating a second near-field light-generating element according to the present invention is characterized in that said step of selecting said minute scattering body includes the steps of: forming a resist film so as to cover at least said minute scattering bodies placed periodically; exposing and developing said resist film; and leaving said one minute scattering body and removing the other minute scattering bodies, using a resist-protecting portion formed by said step of exposing and developing said resist.
According to this invention, the advantages of the method of fabricating the first near-field light-generating element according to the invention can be had. In addition, any special and expensive superaccurate positioning mechanism is not necessary in selecting one out of plural minute scattering bodies. Fabrication equipment used in normal semiconductor processes can be used. Furthermore, if the region in which the periodical plural minute scattering bodies are fabricated is widened, near-field light-generating elements can be fabricated with fabrication equipment having no accurate positioning mechanism. Hence, the cost of the near-field optical head can be reduced further.
A method of fabricating a third near-field light-generating element according to the present invention is characterized in that said step of forming the aperture includes the steps of: forming a light-shielding film on the side of said substrate on which said minute scattering body is formed; and forming the aperture by removing said resist-protecting portion.
According to this invention, the advantages of the method of fabricating the first and second near-field light-generating elements according to the invention can be had. In addition, it is not necessary to form a mask for forming an aperture or to form an aperture by FIB processing, since the resist-protecting portion can be used intact as a mask for forming an aperture. Furthermore, a minute scattering body can be always formed within the aperture without the need to perform a strict alignment or complex fabrication processes. In consequence, the cost of the near-field light-generating element can be reduced further.
A method of fabricating a fourth near-field light-generating element according to the present invention is characterized in that said step of forming the aperture includes the steps of: processing said substrate by thinning parts of said substrate using said resist-protecting portion; forming a light-shielding film on the side of said substrate on which said minute scattering body is formed; and forming the aperture by removing said resist-protecting portion.
According to this invention, the advantages of the method of fabricating the first through third near-field light-generating elements according to the invention can be had. In addition, it is possible to etch the substrate without the need to specially form a mask for etching the substrate. Therefore, near-field light-generating elements can be fabricated at lower cost. Since the light-shielding film and the minute scattering body are equal in height, the distance between the sample and the minute scattering body can be reduced. The intensity of scattering light produced as a result of interaction between the sample surface and the minute scattering body can be increased greatly. This further enhances the efficiency of utilization of light. Additionally, the distance between the sample and the minute scattering body is made quite small, improving the resolution.
A method of fabricating a fifth near-field light-generating element according to the present invention is characterized in that the size of said resist-protecting portion is in excess of the size of said minute scattering body and smaller than the sum of the size of said minute scattering body and the pitch of said minute scattering bodies placed periodically.
According to this invention, the advantages of the method of fabricating the first through fourth near-field light-generating elements according to the invention can be had. In addition, it is unlikely that two or more minute scattering bodies exist in one aperture. It is possible to form one minute scattering body in one aperture. Where a minute scattering body is present inside an aperture, the resolution depends more to the minute scattering body than to the aperture size. Therefore, if two or more minute scattering bodies are formed in one aperture, then the resolution will be deteriorated. However, in the present invention, it is possible to form one minute scattering body in one aperture. Therefore, the resolution of the near-field light-generating element can be prevented from deteriorating. Especially, where the size of the resist-protecting portion is equal to the sum of the size of the minute scattering body and the pitch of the periodically arranged minute scattering bodies, only the minute scattering body always exists inside the resist-protecting portion, even if the accuracy of the position for protecting the resist-protecting portion is as low as several micrometers. As a consequence, a near-field light-generating element can be fabricated with cheap fabrication equipment without the need to strictly form the position of the resist-protecting portion.
A method of fabricating a sixth near-field light-generating element according to the present invention is characterized in that said minute scattering body is gold, silver, copper, or platinum.
According to this invention, the advantages of the method of fabricating the first through fifth near-field light-generating elements according to the invention can be had. In addition, where the minute scattering body is a gold, silver, copper, or platinum, plasmons are easily produced in response to visible light. Furthermore, it is easily available and cheap.